Composite fiber layer construction has been used for the production of composite material articles for the resistance of centrifugal forces, such as in the fabrication of centrifuge rotors. See Piramoon U.S. Pat. No. 4,790,808 issued Dec. 13, 1988 entitled Composite Material Centrifuge Rotor. In that disclosure, particularly at FIG. 2, the construction of disc components is illustrated from unidirectional composite fiber layers. Simply stated, successive sheets of pre-impregnated unidirectional composite fiber layers are placed one on top of another. As each successive layer is placed, the angular orientation of the unidirectional fibers are varied--typically by a preselected angular variation. Thereafter, component article parts for a centrifuge rotor are formed. Excess material is machined off to form discrete discs. Separate discs are assembled from which a centrifuge rotor having quasi-isotropic resistance to centrifugal force can be fabricated. The resistance is said to be quasi-isotropic in that the layers act together to resist centrifugal forces--even though the fibers of one layer may be anisotropic in strength parallel to their axes. Continued fabrication includes individually boring a central hub and sample tube apertures peripheral to the central hub.
Essentially, there are two ways to fabricate a part from composite materials. One approach involves molding the part. Variations in this approach includes Resin Injection Molding (RIM) and in-situ Foam Expansion Fabrication (FEF) which utilize thermoset or thermoplastic formulations, such as PolyEtherlEtherKetone (PEEK), PolyPhenylene Sulfide (PPS), Nylon, etc., to form the structure of a product within a woven or braided fabric placed inside a metal mold. So far, high cost, quality, handling and safety issues have limited molding approach to a handful of high-margin specialty applications such as helicopter blade fabrication.
The other method involves dry or resin coated material which is unidirectional tape or woven fabric. In this approach, individual strips of material are fabricated one on top of another to form a solid, composite article with fibers arrayed in different directions in different layers. The stacked fibers are then cured into a unitary mass. Once in a unitary mass, machining of the mass occurs--both on the exterior and the interior--to produce the finished article. In the case of a centrifuge rotor, the outside is finished to a smooth, rounded profile having low windage, while the inside is machined to form apertures for the sample tubes and central hub.
Current lamination equipment, consisting of a lay-up table and a laser or ultrasound cutter poses several problems. These problems include high cost, slowness of the serial motion of the cutter, damaged fiber-ends and premature curing of the resin on the edges.
Regarding such premature curing of the resin, it has been found that heat from the laser or ultrasound cures resin pre-impregnated fiber in the vicinity of the cut. Subsequently, when the multiple layers are laid, and thereafter cured into a unitary mass for the formation of all or part of a rotor, the partially cured edges do not form into the remaining mass of the rotor. The mass of prematurely cured resin and damaged fibers at the edges are typically machined away during the machining of the finished product. Unfortunately, prematurely cured resin, damaged fibers and the presence of the uncured product complicates the machining.
Heretofore, the bulk of the lay-up work in the industry has been done manually. Hand lay-up increases chances of quality problems, potential for contamination, and also adds significant labor content to the final cost of manufacturing.
These hand lay-up problems are further complicated when the desired part contains cavities, such as a hub aperture or sample tube apertures within a centrifuge rotor or cavities for holding magnets within a high speed electric motor. Heretofore, such apertures, together with the outside surface of the rotor have been machined from so-called "billets"--typically masses of the sequentially layered unidirectional composite fiber tape having many rough edges protruding from a cured, central and unitary mass. Machining such a billet to either form cavities within the final composite structure or to finish the outside surface of the composite structure is a highly specialized and rarely practiced art. Unfortunately, this process can and does damage fibers, lower structural integrity and increase content of labor to produce the final composite material part. In addition, disposing of unusable scrap fibers or powder is a serious environmental issue.